1. Field of the Invention
The present invention relates to a biologically active glass and glass-ceramic composition and an implant coated with the composition.
2. Description of the Prior Art
In the art there are known biologically active glass and glass ceramics which are able chemically to combine directly with hard tissues, in particular, bones in a living body. Upon contacting the body fluids in vivo, the biologically active material reacts and bonds to bones. The mechanism of the reaction between the material and the hard tissue in a living body is described in detail in a publication, J. Biomed. Mater. Res. Symp. No. 2 (Part 1), 117-141 (1971). In summary, the mechanism is as follows:
Constituents of the material such as Na, Ca, P, B and Si are dissolved out from the surface of the material into the body fluid as the respective ions thereby forming micro pores on the surface. P and Ca ions derived from the biologically active material and also P and Ca ions derived from the living body itself owing to its bone making ability are gradually deposited in the formed pores and crystallized into hydroxylapatite which is the main substance of bone. Thus, a direct and chemical bonding is obtained between the biologically active material and the bone.
On the other hand, it is known that no practically useful implant can be produced using only such a biologically active material because of its insufficient mechanical strength. For a solution to the problem, the employment of a metal core having a high mechanical strength sufficient to resist the load normally applied thereto in use, with a coating of such biologically active glass or glass ceramics, has been proposed, and clinical tests have been made for implants thus fabricated.
To apply the coating of biologically active material to the metal core there have been used various methods in which the biologically active glass and glass ceramics have to be molten as a rule. In this case, a preferred coating process is an enamelling process which enables a uniform coating to be realized. However, use of the enamelling process is allowed only when the biologically active material and the metal core to be coated with the material have nearly equal coefficients of thermal expansion and the biologically active material has a relatively low melting point. If the biologically active material and the metal core have different coefficients of thermal expansion, then cracks will easily be developed in the coating layer after cooling. Also, if the biologically active material is not fusible at relatively low temperatures, then the high temperature molten glass or glass ceramics may damage the core and furthermore, the coating may be contaminated with metal ions.
From the above it is concluded that biologically active glass and glass ceramics to be applied to a metal core to form an implant are required to satisfy the following requirements all at once:
(1) To have an adequate reactivity to allow leaching of various ions from its surface; PA1 (2) To have a coefficient of thermal expansion substantially equal to that of the metal core and PA1 (3) To have a relatively low melting point.
All of the known biologically active glass and glass ceramics can not satisfy the above requirements all at once. For example, mention may be made of those biologically active glass and glass ceramics as disclosed in Japanese Patent Application Laid Open No. 145,394/1978 the counterparts of which are U.S. Pat. Nos. 4,159,358 and 4,234,972. Within the range of ingredient contents allowable for composing the biologically active material specified therein, it is not possible to obtain many combinations of the reactivity and coefficient of thermal expansion. In more detail, if the composition of the biologically active material is selected in such manner as to give the material a coefficient of thermal expansion substantially equal to that of the metal core to be coated with the material, then the reactivity of the material is determined thereby. It is no longer possible to determine the reactivity independently of the selected coefficient of thermal expansion.
When one wishes to form an implant by coating a metal core with any known biologically active material, the coefficient of thermal expansion of the coating material must first be determined, considering the coefficient of thermal expansion of the core. Since, as noted above, the reactivity of the material is determined by the selected coefficient of thermal expansion, there is left almost no possibility of free selection of the reactivity after the selection of coefficient of thermal expansion for biologically active glass and glass ceramics hitherto known. However, selection of reactivity is of great importance for usefulness of a biologically active implant as will be seen from the following description.
We, the inventors of the present invention, have conducted a number of experiments on the biologically active glass and glass ceramics proposed by the prior invention, that is, the aforementioned Japanese Patent Application Laid Open No. 145,394/1978 to examine the strength of chemical bonding between the material and various bones. In these experiments the vivo, the known biologically active materials were implanted in thighbones of rats, craniums of rabbits and jawbones of dogs. The results obtained from these animal experiments demonstrated the fact that the chemical bonding strength between the biologically active material and bones is variable according to the kind of test animal and also, even in the same kind of animal, according to age, and condition of the animal and also the position where implantation was made. This difference in the bonding strength obviously resulted from the difference in bone forming ability between different living bodies and also between different parts of the body.
As previously described, when the biologically active material is implanted in a living body, there takes place on the surface of the material a chemical reaction by which ions are leached from the surface and thereby micro pores are formed on the surface. With the proceeding of the surface reaction, new bone is formed owing to the bone forming ability of the living body and the micro pores in the material are gradually filled up with the new bone. A perfect and strong biologically active material-to-bone bond can be attained only when the surface reaction proceeds at a speed substantially equal to the bone making speed. Therefore, in case that the reactivity of the used biologically active material can not follow the bone forming ability of the body part where the material was implanted, it will result in poor bonding strength.
Accordingly, for clinical use of an implant having a coating of biologically active glass or glass ceramics, it is essential to employ such biologically active material, the reactivity of which corresponds to the bone forming ability of the body part where the implant is to be implanted.
As a result of the aforementioned surface reaction of the biologically active material in vivo, ions such as Na, Ca, P, B and Si are leached from the surface. The amount of leaching ions is predominant in Na and Ca. Leaching of other ions such as P, B and Si is gradual and begins after the glass structure has been destroyed to some extent as a result of leaching of Na and Ca ions. This means that evaluation of the reactivity of a biologically active material to a living body can be conducted simply by observing the change of pH primarily attributable to the leached Na ion. More particularly, the material is brought into a simulated physiological solution particularly prepared for this purpose and it is held immersed in the solution long enough to observe the change in pH of the solution resulting mainly from the leaching of Na ion from the material in the solution. In this manner, the evaulation of reactivity of the material to a living body can be performed by a simple pH test in vivo.